The present invention relates to disc prosthesis, and more particularly, to a modular disc prosthesis which is effective to accommodate a variety of vertebral body endplates.
Advancing age, as well as injuries, can lead to changes in the various bones, discs, joints and ligaments of the body. In particular, these changes can manifest themselves in the form of damage or degeneration of an intervertebral disc, the result of which is mild to severe chronic back pain. Intervertebral discs serve as xe2x80x9cshockxe2x80x9d absorbers for the spinal column, absorbing pressure delivered to the spinal column. Additionally, they maintain the proper anatomical separation between two adjacent vertebra. This separation is necessary for allowing both the afferent and efferent nerves to exit and enter, respectively, the spinal column.
Treatment for a diseased or damaged disc can involve the removal of the affected disc and subsequent fusion of the opposing vertebra to one another. Spinal fusion consists of fusing the adjacent vertebrae through the disc space (the space previously occupied by the spinal disc interposed between the adjacent vertebral bodies). Typically, a fusion cage and/or bone graft is placed into the disc space to position the vertebrae apart so as to create more space for the nerves, to restore the angular relationship between the adjacent vertebrae to be fused, and to provide for material that can participate in and promote the fusion process.
In general, the ability to achieve bone fusion appears to be related to certain factors, such as the quality and quantity of bone graft material present, the surface area available for the fusion to occur over, and the stability of the construct being fused. The fusion cage and/or bone graft should, for example, occupy a significant portion of the disc space to provide a large surface area over which fusion can occur, and should contour the vertebral endplates adjacent the disc space to provide stability and further promote fusion. The fusion cage and/or bone graft used for the purpose of interbody fusion, however, cannot always be shaped to precisely fit the complex contours of the vertebral endplates adjacent the disc space. Moreover, the process of preparing and shaping an implant to contour adjacent vertebral endplates can be very time consuming.
Rather than shaping the fusion cage to contour the disc space, procedures have been developed to remove at least a portion of the outermost layer of the vertebral endplates. These procedures, however, can also present the surgeon with several challenges. The vertebral endplates should be prepared to match the implant to provide the greatest possible interface congruity between the endplates and the implant, as well as provide for the optimal contact surface, enhanced fusion area, and enhanced graft and construct stability. In order to achieve this, the amount of bone removed must be to a specified depth and width. Excess removal or penetration of the vertebral endplate can result in a weakening of the structural integrity of the vertebrae, thereby potentially causing the vertebral bodies to collapse around the fusion implant. Moreover, if the shape of the vertebral endplates does not match the shape of the implant, shifting can occur resulting in misalignment of the vertebrae.
Accordingly, there is a need for an implant which allows greater modularity to accommodate a variety of vertebral body endplates, without requiring the vertebral endplates to be prepared or the implant to be shaped and prepared during the procedure.
The present invention provides a modular implant for promoting fusion of adjacent vertebrae to the implant, and optionally provides a shock-absorbing function. The modularity of the implant allows the surgeon to select components which have a shape and size that conforms to the complex contours of the vertebral endplates adjacent the disc space. In general, the implant includes a central core member having superior and inferior surfaces, a superior endplate member having a bone-contacting surface and a mating surface effective to mate with the superior surface of the central core member, and an inferior endplate member having a bone-contacting surface and a mating surface effective to mate with the inferior surface of the central core member. The superior and inferior endplate members can be adapted to fixedly engage the central core member.
The superior and inferior surfaces of the central core member can each include an attachment member adapted to mate with the superior and inferior endplate members, respectively. The attachment members can be mated to the central core member, and can be slidably matable with the endplate members. The modular implant can further include a connecting element disposed on the superior and inferior attachment members and effective to slidably mate the superior and inferior attachment members to the superior and inferior endplate members. In one aspect, the implant includes a posterior portion and an anterior portion, and the superior and inferior endplate members are slidably matable with the superior attachment members in the posterior and anterior directions or, alternatively, in a posterio-lateral or anterio-lateral direction.
In one embodiment, the connecting element is a first complementary member formed on each of the superior and inferior attachment members, and a second complementary member formed on each of the endplate members. The first complementary member, e.g., a female dovetail, is slidably matable with the second complementary member, e.g., a male dovetail. In another embodiment, the connecting element can be formed from at least one protruding element formed on one of the attachment members and the endplate members, and at least one bore formed on the other one of the attachment members and the endplate members. The protruding element is adapted to be disposed within the bore to mate each endplate member to the attachment members of the central core.
The modular implant can optionally include a locking mechanism formed on at least one of the attachment members and the endplate members. The locking mechanism is effective to fixedly engage the superior attachment member to the superior endplate member and the inferior attachment member to the inferior endplate member. In one embodiment, the locking mechanism is a bore which extends through the complementary male and female dovetail complements, and a pin slidably disposed through the bore. The pin is effective to prevent movement of the attachments members with respect to the endplate members. In another embodiment, the locking mechanism can be a bore which extends through the attachment member and the endplate member, and a pin member adapted to be disposed through the bore.
The modular implant system of the invention can include a plurality of superior and inferior endplate members. The bone-contacting surfaces of the various superior and inferior endplate members can each have specific geometries and sizes adapted to conform to vertebral body endplates of various shapes and sizes. The geometries of the superior and inferior endplate members can be, for example, a convex shape, a domed shape, a serpentine shape, a ramped shape, and an angled shape, and combinations thereof. This type of modular system allows surgeons to construct an implant having superior and inferior endplate members which are adapted to fit within the vertebral space.
The present invention also provides a method for inserting an implant between a superior vertebrae and an inferior vertebrae.